1. Field of the Invention
The present invention relates to a control system for an internal combustion engine, and particularly to a control system for an internal combustion engine in which fuel containing alcohol is used.
2. Description of the Related Art
Japanese Patent Laid-open No. H3-253733 (JP-'733) discloses a method for controlling a throttle valve opening in an internal combustion engine in which alcohol, gasoline, or composite fuel of alcohol and gasoline (gasohol) is used. According to this control method, a target throttle valve opening is set according to a concentration of alcohol in the fuel, a depression amount of an accelerator pedal, and an engine rotational speed, so that constant drivability of the engine (engine output) is obtained regardless of the concentration of alcohol in the fuel. Further, an actual throttle valve opening is controlled to coincide with the target throttle valve opening.
Further, Japanese Utility Model Laid-open No. H2-20742 (JP-'742) discloses a throttle valve opening adjusting device for an internal combustion engine in which gasoline is used as fuel. According to this device, a maximum throttle valve opening is set according to an engine rotational speed and an engine coolant temperature, and a target throttle valve opening, which is set according to the depression amount of the accelerator pedal, is limited to a value which is equal to or less than the maximum throttle valve opening. According to such limitation, the combustion state of the engine is prevented from deteriorating especially when the engine temperature is low.
In the internal combustion engine in which fuel containing alcohol is used, the combustion state may deteriorate with high possibility especially when the accelerator pedal is greatly depressed immediately after the cold start of the engine and the throttle valve is opened according to the depression amount. In the control method of JP-'733, suppressing deterioration of the drivability of the engine is taken into consideration. However, a problem of deterioration of the combustion state is not taken into consideration. Further, in the device of JP-'742, the engine coolant temperature is taken into consideration, which is, however, not sufficient for preventing the combustion state from deteriorating.